Resonance tuning module for implantable devices and leads

ABSTRACT

An implantable medical assist device includes a medical device. The medical device has a housing and electronics contained therein. A lead provides an electrical path to or from the electronics within the medical device. A resonance tuning module is located in the housing and is connected to the lead. The resonance tuning module includes a control circuit for determining a resonant frequency of the implantable medical assist device and an adjustable impedance circuit to change the combined resonant frequency of the medical device and lead.

The present application is a divisional application of U.S. patentapplication Ser. No. 11/696,857, titled “RESONANCE TUNING MODULE FORIMPLANTABLE DEVICES AND LEADS” and filed Apr. 5, 2007, which claims thebenefit of: U.S. Provisional Patent Application, Ser. No. 60/744,468,titled “RESONANCE TUNING MODULE FOR IMPLANTABLE DEVICES AND LEADS” andfiled on Apr. 7, 2006; U.S. Provisional Patent Application, Ser. No.60/806,115, titled “RESONANCE TUNING MODULE FOR IMPLANTABLE DEVICES ANDLEADS” and filed on Jun. 29, 2006; U.S. Provisional Patent Application,Ser. No. 60/744,464, titled “MODULAR RESONANCE COMPONENT FOR TISSUEINVASIVE DEVICES” and filed on Apr. 7, 2006; and U.S. Provisional PatentApplication, Ser. No. 60/747,027, titled “MODULAR RESONANCE COMPONENTFOR TISSUE INVASIVE DEVICES” and filed on May 11, 2006.

The entire contents of U.S. Pat. Nos. 6,829,509 and 6,949,929 are herebyincorporated by reference. The entire contents of U.S. patentapplication Ser. Nos. 11/696,857; 11/214,640; 10/972,275; 10/077,906;10/780,261; and 10/887,533 are hereby incorporated by reference. Theentire contents of U.S. Provisional Patent Application Ser. No.60/744,468, filed on Apr. 7, 2006; U.S. Provisional Patent ApplicationSer. No. 60/806,115, filed on Jun. 29, 2006; U.S. Provisional PatentApplication Ser. No. 60/744,464, filed on Apr. 7, 2006; and U.S.Provisional Patent Application Ser. No. 60/747,027, filed on May 11,2006 are hereby incorporated by reference.

FIELD OF THE PRESENT INVENTION

The present invention relates generally to a medical device thatincludes an anti-antenna device to prevent or significantly reducedamaging heat, created by currents or voltages induced by outsideelectromagnetic energy, to a tissue area. More particularly, the presentinvention is directed to a medical device that includes an anti-antennadevice to prevent or significantly reduce damaging heat, created bycurrents or voltages induced by magnetic-resonance imaging, to a tissuearea.

BACKGROUND OF THE PRESENT INVENTION

Magnetic resonance imaging has been developed as an imaging techniqueadapted to obtain both images of anatomical features of human patientsas well as some aspects of the functional activities of biologicaltissue. These images have medical diagnostic value in determining thestate of the health of the tissue examined.

In a magnetic-resonance imaging process, a patient is typically alignedto place the portion of the patient's anatomy to be examined in theimaging volume of the magnetic-resonance imaging apparatus. Such amagnetic-resonance imaging apparatus typically comprises a primarymagnet for supplying a constant magnetic field (B₀) which, byconvention, is along the z-axis and is substantially homogeneous overthe imaging volume and secondary magnets that can provide linearmagnetic field gradients along each of three principal Cartesian axes inspace (generally x, y, and z, or x₁, x₂ and x₃, respectively). Amagnetic field gradient (ΔB₀/Δx_(i)) refers to the variation of thefield along the direction parallel to B₀ with respect to each of thethree principal Cartesian axes, x_(i). The apparatus also comprises oneor more radio-frequency coils, which provide excitation signals to thepatient's body, placed in the imaging volume in the form of a pulsedrotating magnetic field. This field is commonly referred to as thescanner's “B1” field and as the scanner's “RF” or “radio-frequency”field. The frequency of the excitation signals is the frequency at whichthis magnetic field rotates. These coils may also be used for detectionof the excited patient's body material magnetic-resonance imagingresponse signals.

The use of the magnetic-resonance imaging process with patients who haveimplanted medical assist devices; such as cardiac assist devices orimplanted insulin pumps; often presents problems. As is known to thoseskilled in the art, implantable devices (such as implantable pulsegenerators and cardioverter/defibrillator/pacemakers) are sensitive to avariety of forms of electromagnetic interference because theseenumerated devices include sensing and logic systems that respond tolow-level electrical signals emanating from the monitored tissue regionof the patient. Since the sensing systems and conductive elements ofthese implantable devices are responsive to changes in localelectromagnetic fields, the implanted devices are vulnerable to externalsources of severe electromagnetic noise, and in particular, toelectromagnetic fields emitted during the magnetic resonance imagingprocedure. Thus, patients with implantable devices are generally advisednot to undergo magnetic resonance imaging procedures.

The human heart may suffer from two classes of rhythmic disorders orarrhythmias: bradycardia and tachyarrhythmia. Bradycardia occurs whenthe heart beats too slowly, and may be treated by a common implantablepacemaker delivering low voltage (about 3 Volts) pacing pulses.

The common implantable pacemaker is usually contained within ahermetically sealed enclosure, in order to protect the operationalcomponents of the device from the harsh environment of the body, as wellas to protect the body from the device.

The common implantable pacemaker operates in conjunction with one ormore electrically conductive leads, adapted to conduct electricalstimulating pulses to sites within the patient's heart, and tocommunicate sensed signals from those sites back to the implanteddevice.

Furthermore, the common implantable pacemaker typically has a metal caseand a connector block mounted to the metal case that includesreceptacles for leads which may be used for electrical stimulation orwhich may be used for sensing of physiological signals. The battery andthe circuitry associated with the common implantable pacemaker arehermetically sealed within the case. Electrical interfaces are employedto connect the leads outside the metal case with the medical devicecircuitry and the battery inside the metal case.

Electrical interfaces serve the purpose of providing an electricalcircuit path extending from the interior of a hermetically sealed metalcase to an external point outside the case while maintaining thehermetic seal of the case. A conductive path is provided through theinterface by a conductive pin that is electrically insulated from thecase itself.

Such interfaces typically include a ferrule that permits attachment ofthe interface to the case, the conductive pin, and a hermetic glass orceramic seal that supports the pin within the ferrule and isolates thepin from the metal case.

A common implantable pacemaker can, under some circumstances, besusceptible to electrical interference such that the desiredfunctionality of the pacemaker is impaired. For example, commonimplantable pacemaker requires protection against electricalinterference from electromagnetic interference or insult, defibrillationpulses, electrostatic discharge, or other generally large voltages orcurrents generated by other devices external to the medical device. Asnoted above, more recently, it has become crucial that cardiac assistsystems be protected from magnetic-resonance imaging sources.

Such electrical interference can damage the circuitry of the cardiacassist systems or cause interference in the proper operation orfunctionality of the cardiac assist systems. For example, damage mayoccur due to high voltages or excessive currents introduced into thecardiac assist system.

Therefore, it is required that such voltages and currents be limited atthe input of such cardiac assist systems, e.g., at the interface.Protection from such voltages and currents has typically been providedat the input of a cardiac assist system by the use of one or more zenerdiodes and one or more filter capacitors.

For example, one or more zener diodes may be connected between thecircuitry to be protected, e.g., pacemaker circuitry, and the metal caseof the medical device in a manner, which grounds voltage surges andcurrent surges through the diode(s). Such zener diodes and capacitorsused for such applications may be in the form of discrete componentsmounted relative to circuitry at the input of a connector block wherevarious leads are connected to the implantable medical device, e.g., atthe interfaces for such leads.

However, such protection, provided by zener diodes and capacitors placedat the input of the medical device, increases the congestion of themedical device circuits, at least one zener diode and one capacitor perinput/output connection or interface. This is contrary to the desire forincreased miniaturization of implantable medical devices.

Further, when such protection is provided, interconnect wire length forconnecting such protection circuitry and pins of the interfaces to themedical device circuitry that performs desired functions for the medicaldevice tends to be undesirably long. The excessive wire length may leadto signal loss and undesirable inductive effects. The wire length canalso act as an antenna that conducts undesirable electrical interferencesignals to sensitive CMOS circuits within the medical device to beprotected.

Additionally, the radio-frequency energy that is inductively coupledinto the wire causes intense heating along the length of the wire, andat the electrodes that are attached to the heart wall. This heating maybe sufficient to ablate the interior surface of the blood vessel throughwhich the wire lead is placed, and may be sufficient to cause scarringat the point where the electrodes contact the heart. A further result ofthis ablation and scarring is that the sensitive node that the electrodeis intended to pace with low voltage signals becomes desensitized, sothat pacing the patient's heart becomes less reliable, and in some casesfails altogether.

Another conventional solution for protecting the implantable medicaldevice from electromagnetic interference is illustrated in FIG. 1. FIG.1 is a schematic view of an implantable medical device 12 embodyingprotection against electrical interference. At least one lead 14 isconnected to the implantable medical device 12 in connector block region13 using an interface.

In the case where implantable medical device 12 is a pacemaker implantedin a body 10, the pacemaker 12 includes at least one or both of pacingand sensing leads represented generally as leads 14 to sense electricalsignals attendant to the depolarization and repolarization of the heart16, and to provide pacing pulses for causing depolarization of cardiactissue in the vicinity of the distal ends thereof.

Conventionally protection circuitry is provided using a diode arraycomponent. The diode array conventionally consists of five zener diodetriggered semiconductor controlled rectifiers with anti-parallel diodesarranged in an array with one common connection. This allows for a smallfootprint despite the large currents that may be carried through thedevice during defibrillation, e.g., 10 amps. The semiconductorcontrolled rectifiers turn ON and limit the voltage across the devicewhen excessive voltage and current surges occur.

Each of the zener diode triggered semiconductor controlled rectifier isconnected to an electrically conductive pin. Further, each electricallyconductive pin is connected to a medical device contact region to bewire bonded to pads of a printed circuit board. The diode arraycomponent is connected to the electrically conductive pins via the diecontact regions along with other electrical conductive traces of theprinted circuit board.

Other attempts have been made to protect implantable devices frommagnetic-resonance imaging fields. For example, U.S. Pat. No. 5,968,083describes a device adapted to switch between low and high impedancemodes of operation in response to electromagnetic interference orinsult. Furthermore, U.S. Pat. No. 6,188,926 discloses a control unitfor adjusting a cardiac pacing rate of a pacing unit to an interferencebackup rate when heart activity cannot be sensed due to electromagneticinterference or insult.

Although, conventional medical devices provide some means for protectionagainst electromagnetic interference, these conventional devices requiremuch circuitry and fail to provide fail-safe protection againstradiation produced by magnetic-resonance imaging procedures. Moreover,the conventional devices fail to address the possible damage that can bedone at the tissue interface due to radio-frequency induced heating, andthe conventional devices fail to address the unwanted heart stimulationthat may result from radio-frequency induced electrical currents.

Thus, it is desirable to provide devices that prevent the possibledamage that can be done at the tissue interface due to inducedelectrical signals that may cause thermally-related tissue damage.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may take form in various components andarrangements of components, and in various steps and arrangements ofsteps. The drawings are only for purposes of illustrating a preferredembodiment and are not to be construed as limiting the presentinvention, wherein:

FIG. 1 is an illustration of conventional cardiac assist device;

FIG. 2 shows a conventional bipolar pacing lead circuit representation;

FIG. 3 is a graph illustrating the magnitude of the current, induced bymagnetic-resonance imaging, flowing through the tissue at a distal endof a medical device using the bipolar pacing lead circuit of FIG. 2;

FIG. 4 shows a bipolar pacing lead circuit representation according tosome or all of the concepts of the present invention;

FIG. 5 is a graph illustrating the magnitude of the current, induced bymagnetic-resonance imaging, flowing through the tissue at a distal endof a medical device using the bipolar pacing lead circuit of FIG. 4;

FIG. 6 is a graph illustrating the magnitude of the current,low-frequency pacing or defibrillation signals, flowing through thecircuit of a medical device using the bipolar pacing lead circuit ofFIG. 4;

FIG. 7 shows another bipolar pacing lead circuit representationaccording to some or all of the concepts of the present invention;

FIG. 8 is a graph illustrating the magnitude of the current, induced by64 MHz magnetic-resonance imaging, flowing through the tissue at adistal end of a medical device using the bipolar pacing lead circuit ofFIG. 7;

FIG. 9 is a graph illustrating the magnitude of the current, induced by128 MHz magnetic-resonance imaging, flowing through the tissue at adistal end of a medical device using the bipolar pacing lead circuit ofFIG. 7;

FIG. 10 shows another bipolar pacing lead circuit representationaccording to some or all of the concepts of the present invention;

FIG. 11 is a graph illustrating the magnitude of the current, induced bymagnetic-resonance imaging, flowing through the tissue at a distal endof a medical device using the bipolar pacing lead circuit of FIG. 10;

FIG. 12 shows another bipolar pacing lead circuit representationaccording to some or all of the concepts of the present invention;

FIG. 13 is a graph illustrating the magnitude of the current, induced bymagnetic-resonance imaging, flowing through the tissue at a distal endof a medical device using the bipolar pacing lead circuit of FIG. 12;

FIG. 14 shows another bipolar pacing lead circuit representationaccording to some or all of the concepts of the present invention;

FIG. 15 is a graph illustrating the magnitude of the current, induced bymagnetic-resonance imaging, flowing through the tissue at a distal endof a medical device using the bipolar pacing lead circuit of FIG. 14;

FIG. 16 is a graph illustrating the magnitude of the current, induced bymagnetic-resonance imaging, flowing through the tissue at a distal endof a medical device using the bipolar pacing lead circuit of FIG. 14using an increased resistance in the resonant circuit;

FIG. 17 is a graph illustrating the magnitude of the current, induced bymagnetic-resonance imaging, flowing through the tissue at a distal endof a medical device using a conventional bipolar pacing lead circuit;

FIG. 18 is a graph illustrating the magnitude of the current, induced bymagnetic-resonance imaging, flowing through the tissue at a distal endof a medical device using the bipolar pacing lead circuit with aresonant circuit in one lead;

FIG. 19 is a graph illustrating the magnitude of the current, induced bymagnetic-resonance imaging, flowing through the tissue at a distal endof a medical device using the bipolar pacing lead circuit with aresonant circuit in both leads;

FIG. 20 is a graph illustrating the magnitude of the current, induced bymagnetic-resonance imaging, flowing through the tissue at a distal endof a medical device using the bipolar pacing lead circuit with aresonant circuit in both leads and increased inductance;

FIG. 21 is a graph illustrating the magnitude of the current, induced bymagnetic-resonance imaging, flowing through the tissue at a distal endof a medical device using the bipolar pacing lead circuit with aresonant circuit in both leads and decreased inductance;

FIG. 22 shows a bipolar pacing lead adaptor with a single resonantcircuit according to some or all of the concepts of the presentinvention;

FIG. 23 illustrates a resonant circuit for a bipolar pacing leadaccording to some or all of the concepts of the present invention;

FIG. 24 is a graph illustrating the temperature at a distal end of amedical device;

FIG. 25 is a graph illustrating the magnitude of the current, induced bymagnetic-resonance imaging, flowing through the tissue at a distal endof a conventional medical device;

FIG. 26 is a graph illustrating the magnitude of the current, induced bymagnetic-resonance imaging, flowing through the tissue at a distal endof a medical device with a resonant circuit, according to the conceptsof the present invention, at the proximal end thereof;

FIG. 27 is a graph illustrating the temperature at a distal end of amedical device with a resonant circuit, according to the concepts of thepresent invention, at the distal end thereof;

FIG. 28 is a graph illustrating the temperature at a distal end of amedical device with a resonant circuit, according to the concepts of thepresent invention, at the proximal end thereof;

FIG. 29 shows another bipolar pacing lead circuit representationaccording to some or all of the concepts of the present invention;

FIG. 30 shows a bipolar pacing device according to some or all of theconcepts of the present invention;

FIG. 31 is a graph illustrating the temperature at a distal end of aconventional pacing lead;

FIG. 32 is a graph illustrating the temperature at a distal end of thebipolar pacing device of FIG. 30 with closed windings;

FIG. 33 is a graph illustrating the temperature at a distal end of thebipolar pacing device of FIG. 30 with medium closed windings;

FIG. 34 is a graph illustrating the temperature at a distal end of thebipolar pacing device of FIG. 30 with open pitch windings;

FIG. 35 is a graph illustrating the temperature at a distal end of thebipolar pacing device of FIG. 30 with different states of windings;

FIG. 36 shows a resonance tuning module as an integral subsystem in adevice such as a pacemaker according to some or all of the concepts ofthe present invention;

FIG. 37 shows a resonance tuning module as a secondary module that maybe interposed between the device and its associated leads or leadsaccording to some or all of the concepts of the present invention;

FIG. 38 illustrates a conventional electronic device that includes aconnection port, a circuit, and an electrical line;

FIG. 39 illustrates an insertion of resonant circuit adaptor suitablefor connecting to connector and connecting to connection port whichplaces a resonant circuit in series with electrical wire and circuit;

FIG. 40 illustrates another electronic device that includes a connectionport and a circuit;

FIG. 41 illustrates an electronic device having a circuit, a connectionport, and a switch;

FIG. 42 illustrates components of an RLC resonance circuit connected toan electrode;

FIGS. 43-45 illustrate embodiments of a cylinder shaped componentimplemented with a resonance circuit;

FIG. 46 illustrates a plurality of cylinder shaped components positionedalong a medical device;

FIG. 47 illustrates a cross-sectional view of an embodiment of acylinder shaped component implemented with a resonance circuit;

FIG. 48 illustrates a cross-sectional view of another embodiment of acylinder shaped component implemented with a resonance circuit;

FIG. 49 illustrates a lead with an RLC circuit at distal tip tuned todesired resonant frequency;

FIG. 50 illustrates a lead with multiple RLC circuits at distal end oflead;

FIG. 51 illustrates a lead with multiple RLC circuits positioned alongthe leads length;

FIG. 52 illustrates a diode is placed at the distal end of the lead;

FIG. 53 illustrates an RLC circuit, as illustrated in FIG. 49, whereintwo diodes are connected back to back across an inductor;

FIG. 54 illustrates an RLC circuit, as illustrated in FIG. 49, whereintwo diodes are connected back to back across a capacitor; and

FIG. 55 illustrates the impedance of a coiled wire of the lead beingdivided into two sections.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

As noted above, a medical device includes an anti-antenna device toprevent or significantly reduce damaging heat, created by currents orvoltages induced by outside electromagnetic energy (namelymagnetic-resonance imaging), to a tissue area.

More specifically, the present invention is directed to a medical devicethat includes anti-antenna device, which significantly reduces theinduced current on the “signal” wire of a pacing lead when the pacinglead is subjected to the excitation signal's frequency of amagnetic-resonance imaging scanner without significantly altering a lowfrequency pacing signal. The low frequency pacing signal may begenerated by an implantable pulse generator or other pulse generatorsource outside the body.

To provide an anti-antenna device, in one embodiment of the presentinvention utilizes a resonant circuit or circuits in line with a lead.The lead may be a signal wire of the pacing lead. Although the followingdescriptions of the various embodiments of the present invention, aswell as the attached claims may utilize, the term pacing lead or lead,the term pacing lead or lead may generically refer to a unipolar pacinglead having one conductor; a bipolar pacing lead having two conductors;an implantable cardiac defibrillator lead; a deep brain stimulating leadhaving multiple conductors; a nerve stimulating lead; and/or any othermedical lead used to deliver an electrical signal to or from a tissuearea of a body. The resonant circuit or circuits provide a blockingquality with respect to the currents induced by the excitation signal'sfrequency of the magnetic-resonance imaging scanner. The excitationsignal's frequency of the magnetic-resonance imaging scanner is commonlydefined as the rotational frequency of the scanner's excitation magneticfield, commonly known as the scanner's B1 field.

FIG. 2 provides a conventional circuit representation of a bipolarpacing lead. As illustrated in FIG. 2, the bipolar pacing lead 1000includes two leads (100 and 200). A first pacing lead 100 includesresistance and inductance represented by a first resistor 120 and afirst inductor 110, respectively. A second pacing lead 200 includesresistance and inductance represented by a second resistor 220 and asecond inductor 210, respectively. At a distal end of each lead, theleads (100 and 200) come in contact with tissue.

As illustrated in FIG. 2, the circuit paths from the distal ends of theleads (100 and 200) include a first tissue resistance, represented byfirst tissue modeled resistor 130, and a second tissue resistance,represented by second tissue modeled resistor 230.

The conventional circuit representation of a bipolar pacing lead, asillustrated in FIG. 2, further includes a voltage source 300 thatrepresents the induced electromagnetic energy (voltage or current) frommagnetic resonance imaging, a body modeled resistor 400 that representsthe resistance of the body, and a differential resistor 500 thatrepresents a resistance between the leads.

In FIG. 3, it is assumed that the bipolar pacing leads of FIG. 2 aresubjected to a 64 MHz magnetic resonance imaging environment. Asdemonstrated in FIG. 3, the current induced by the 64 MHz magneticresonance imaging environment and flowing through the tissue at thedistal end of the bipolar pacing leads can have a magnitude between 0.85and −0.85 amps. This magnitude of current (IRt2, which represents thecurrent flowing through first tissue modeled resistors 130 and IRt1,which represents the current flowing through second tissue modeledresistors 230) at the distal end of the bipolar pacing leads can lead toserious damage to the tissue due to heat generated by the currentflowing to the tissue.

To reduce the heat generated by the induced current in the tissue, FIG.4 provides a circuit representation of a bipolar pacing lead accordingto the concepts of the present invention. As illustrated in FIG. 4, thebipolar pacing lead 1000 includes two leads (1100 and 1200). A firstpacing lead 1100 includes resistance and inductance represented by afirst resistor 1120 and a first inductor 1110, respectively. A secondpacing lead 1200 includes resistance and inductance represented by asecond resistor 1220 and a second inductor 1210, respectively. At adistal end of each lead, the leads (1100 and 1200) come in contact withtissue.

As illustrated in FIG. 4, the circuit paths from the distal ends of theleads (1100 and 1200) include a first tissue resistance, represented byfirst tissue modeled resistor 1130, and a second tissue resistance,represented by second tissue modeled resistor 1230.

The circuit representation of a bipolar pacing lead, as illustrated inFIG. 4, further includes a voltage source 300 that represents theinduced electromagnetic energy (voltage or current) from magneticresonance imaging, a body modeled resistor 400 that represents theresistance of the body, and a differential resistor 500 that representsa resistance between the leads.

In addition to the elements discussed above, the circuit representationof a bipolar pacing lead, as illustrated in FIG. 4, includes a resonantcircuit 2000 in series or inline with one of the pacing leads, namelythe second lead 1200. The resonant circuit 2000 includes a LC circuithaving an inductor 2110 in parallel to a capacitor 2120. The resonantcircuit 2000, together with the second lead 1200, acts as ananti-antenna device, thereby reducing the magnitude of the currentinduced through the tissue at the distal end of the pacing leads (1100and 1200).

In FIG. 5, it is assumed that the bipolar pacing leads of FIG. 4 aresubjected to a 64 MHz magnetic resonance imaging environment. Asdemonstrated in FIG. 5, the current (IRt2, which represents the currentflowing through tissue modeled resistor 1230 of FIG. 4) induced by the64 MHz magnetic resonance imaging environment and flowing through thetissue at the distal end of the second bipolar pacing lead 1200 can begreatly reduced. It is noted that the current (IRt1, which representsthe current flowing through tissue modeled resistor 1130 of FIG. 4)induced by the 64 MHz magnetic resonance imaging environment and flowingthrough the tissue at the distal end of the second bipolar pacing lead1100, according to this simulated model, can have a magnitude between1.21 and −1.21 amps. This reduced magnitude of current (IRt2, whichrepresents the current flowing through tissue modeled resistor 1230 ofFIG. 4) at the distal end of the bipolar pacing lead can significantlyreduce the damage to the tissue due to heat generated by the currentflowing to the tissue.

Notwithstanding the inclusion of the resonant circuit 2000, the bipolarpacing leads can still provide an efficient pathway for the pacingsignals, as illustrated by FIG. 6. As can be seen when compared to FIG.3, the current magnitudes shown in FIG. 6 through tissue resistors 1130and 1230, shown in FIG. 4, are approximately the same as the magnitudesof the currents passing through tissue resistors 130 and 230, shown inFIG. 2. Thus, with the resonant circuit 2000 inserted into the circuitof FIG. 4, the low frequency pacing signals are not significantlyaltered.

To provide a further reduction of the heat generated by the inducedcurrent in the tissue, FIG. 7 provides a circuit representation of abipolar pacing lead according to the concepts of the present invention.As illustrated in FIG. 7, the bipolar pacing lead 1000 includes twoleads (1100 and 1200). A first pacing lead 1100 includes resistance andinductance represented by a first resistor 1120 and a first inductor1110, respectively. A second pacing lead 1200 includes resistance andinductance represented by a second resistor 1220 and a second inductor1210, respectively. At a distal end of each lead, the leads (1100 and1200) come in contact with tissue.

As illustrated in FIG. 7, the circuit paths from the distal ends of theleads (1100 and 1200) include a first tissue resistance, represented byfirst tissue modeled resistor 1130, and a second tissue resistance,represented by second tissue modeled resistor 1230.

The circuit representation of a bipolar pacing lead, as illustrated inFIG. 7, further includes a voltage source 300 that represents theinduced electromagnetic energy (voltage or current) from magneticresonance imaging, a body resistor 400 that represents the resistance ofthe body, and a differential resistor 500 that represents a resistancebetween the leads.

In addition to the elements discussed above, the circuit representationof a bipolar pacing lead, as illustrated in FIG. 7, includes tworesonant circuits (2000 and 3000) in series or inline with one of thepacing leads, namely the second lead 1200. The first resonant circuit2000 includes a LC circuit, tuned to about 64 MHz, having an inductor2110 in parallel to a capacitor 2120. The second resonant circuit 3000includes a LC circuit, tuned to about 128 MHz, having an inductor 3110in parallel to a capacitor 3120.

The lead 1200 together with the inline resonant circuits (2000 and 3000)act as an anti-antenna device, thereby reducing the magnitude of thecurrent induced through the tissue at the distal end of the pacing lead(1200).

In FIG. 8, it is assumed that the bipolar pacing leads of FIG. 7 aresubjected to a 64 MHz magnetic resonance imaging environment. Asdemonstrated in FIG. 8, the current (IRt1, which represents the currentflowing through tissue modeled resistor 1230 of FIG. 7) induced by the64 MHz magnetic resonance imaging environment and flowing through thetissue at the distal end of the second bipolar pacing lead 1200 can begreatly reduced. It is noted that the current (IRt2, which representsthe current flowing through tissue modeled resistor 1130 of FIG. 7)induced by the 64 MHz magnetic resonance imaging environment and flowingthrough the tissue at the distal end of the first bipolar pacing lead1100 can have a magnitude between 1.21 and −1.21 amps. This reducedmagnitude of current (IRt1, which represents the current flowing throughtissue modeled resistor 1230 of FIG. 7) at the distal end of the bipolarpacing lead can significantly reduce the damage to the tissue due toheat generated by the current flowing to the tissue.

In FIG. 9, it is assumed that the bipolar pacing leads of FIG. 7 aresubjected to a 128 MHz magnetic resonance imaging environment. Asdemonstrated in FIG. 9, the current (IRt1, which represents the currentflowing through tissue modeled resistor 1230 of FIG. 7) induced by the128 MHz magnetic resonance imaging environment and flowing through thetissue at the distal end of the second bipolar pacing lead 1200 can begreatly reduced. It is noted that the current (IRt2, which representsthe current flowing through tissue modeled resistor 1130 of FIG. 7)induced by the 128 MHz magnetic resonance imaging environment andflowing through the tissue at the distal end of the first bipolar pacinglead 1100 can have a magnitude between 1.21 and −1.21 amps. This reducedmagnitude of current (IRt1, which represents the current flowing throughtissue modeled resistor 1230 of FIG. 7) at the distal end of the bipolarpacing lead can significantly reduce the damage to the tissue due toheat generated by the current flowing to the tissue.

It is noted that by including the two resonant circuits (2000 and 3000),the bipolar pacing leads can reduce heat generation, notwithstanding theoperational frequency of the magnetic resonance imaging scanner. It isnoted that further resonant circuits may be added, each tuned to aparticular operational frequency of a magnetic resonance imagingscanner.

To reduction of the heat generated by the induced current in the tissue,FIG. 10 provides a circuit representation of a bipolar pacing leadaccording to the concepts of the present invention. As illustrated inFIG. 10, the bipolar pacing lead 1000 includes two leads (1100 and1200). A first pacing lead 1100 includes resistance and inductancerepresented by a first resistor 1120 and a first inductor 1110,respectively. A second pacing lead 1200 includes resistance andinductance represented by a second resistor 1220 and a second inductor1210, respectively. At a distal end of each lead, the leads (1100 and1200) come in contact with tissue.

As illustrated in FIG. 10, the circuit paths from the distal ends of theleads (1100 and 1200) include a first tissue resistance, represented byfirst tissue modeled resistor 1130, and a second tissue resistance,represented by second tissue modeled resistor 1230.

The circuit representation of a bipolar pacing lead, as illustrated inFIG. 10, further includes a voltage source 300 that represents theinduced electromagnetic energy (voltage or current) from magneticresonance imaging, a body resistor 400 that represents the resistance ofthe body, and a differential resistor 500 that represents a resistancebetween the leads.

In addition to the elements discussed above, the circuit representationof a bipolar pacing lead, as illustrated in FIG. 10, includes tworesonant circuits (2000 and 3000) in series or inline with one of thepacing leads, namely the second lead 1200. The first resonant circuit2000 includes a LC circuit, tuned to about 64 MHz, having an inductor2110 in parallel to a capacitor 2120. The second resonant circuit 3000includes a LC circuit, tuned to about 128 MHz, having an inductor 3110in parallel to a capacitor 3120.

The lead 1200 together with the inline resonant circuits (2000 and 3000)act as an anti-antenna device, thereby reducing the magnitude of thecurrent induced through the tissue at the distal end of the pacing lead(1200).

Lastly, the circuit representation of a bipolar pacing lead, asillustrated in FIG. 10, includes a capacitance circuit 4000 (a capacitorand resistor), which may represent parasitic capacitance or distributivecapacitance in the second pacing lead (1200) or additional capacitanceadded to the pacing lead. It is noted that the parasitic capacitance ordistributive capacitance is the inherent capacitance in a pacing leadalong its length. Moreover, it is noted that the parasitic capacitanceor distributive capacitance may be the inter-loop capacitance in acoiled wire pacing lead. The location of the capacitance circuit 4000positions the resonant circuits (2000 and 3000) at the proximal end ofthe pacing lead (1200).

In FIG. 11, it is assumed that the bipolar pacing leads of FIG. 10 aresubjected to a magnetic resonance imaging environment having anoperating radio frequency of approximately 64 MHz. As demonstrated inFIG. 11, the current (IRt1, which represents the current flowing throughtissue modeled resistor 1230 of FIG. 10) induced by the magneticresonance imaging environment and flowing through the tissue at thedistal end of the second bipolar pacing lead 1200 is not reduced by thesame amount as the previous circuits. In other words, the capacitancecircuit 4000 lowers the effectiveness of the resonant circuits (2000 and3000), located at the proximal end of the lead, to block the magneticresonance imaging induced currents.

It is noted that the current (IRt2, which represents the current flowingthrough tissue modeled resistor 1130 of FIG. 10) induced by the magneticresonance imaging environment and flowing through the tissue at thedistal end of the first bipolar pacing lead 1100 can have a magnitudebetween 1.21 and −1.21 amps.

Although the resonant circuits (2000 and 3000) still reduce the inducedcurrent, the capacitance circuit 4000 reduces the effectiveness of theresonant circuits (2000 and 3000). To increase the effectiveness of theresonant circuits (2000 and 3000), the resonant circuits (2000 and 3000)are moved to the distal end of the pacing lead, as illustrated in FIG.12.

To reduction of the heat generated by the induced current in the tissue,FIG. 12 provides a circuit representation of a bipolar pacing leadaccording to the concepts of the present invention. As illustrated inFIG. 12, the bipolar pacing lead 1000 includes two leads (1100 and1200). A first pacing lead 1100 includes resistance and inductancerepresented by a first resistor 1120 and a first inductor 1110,respectively. A second pacing lead 1200 includes resistance andinductance represented by a second resistor 1220 and a second inductor1210, respectively. At a distal end of each lead, the leads (1100 and1200) come in contact with tissue.

As illustrated in FIG. 12, the circuit paths from the distal ends of theleads (1100 and 1200) include a first tissue resistance, represented byfirst tissue modeled resistor 1130, and a second tissue resistance,represented by second tissue modeled resistor 1230.

The circuit representation of a bipolar pacing lead, as illustrated inFIG. 12, further includes a voltage source 300 that represents theinduced electromagnetic energy (voltage or current) from magneticresonance imaging, a body resistor 400 that represents the resistance ofthe body, and a differential resistor 500 that represents a resistancebetween the leads.

In addition to the elements discussed above, the circuit representationof a bipolar pacing lead, as illustrated in FIG. 12, includes tworesonant circuits (2000 and 3000) in series or inline with one of thepacing leads, namely the second lead 1200. The first resonant circuit2000 includes a LC circuit, tuned to about 64 MHz, having an inductor2110 in parallel to a capacitor 2120. The second resonant circuit 3000includes a LC circuit, tuned to about 128 MHz, having an inductor 3110in parallel to a capacitor 3120.

The lead 1200 together with the inline resonant circuits (2000 and 3000)act as an anti-antenna device, thereby reducing the magnitude of thecurrent induced through the tissue at the distal end of the pacing leads(1100 and 1200).

Lastly, the circuit representation of a bipolar pacing lead, asillustrated in FIG. 12, includes a capacitance circuit 4000 (a capacitorand resistor), which may represent parasitic capacitance in the secondpacing lead (1200) or additional capacitance added to the pacing lead.The location of the capacitance circuit 4000 positions the resonantcircuits (2000 and 3000) at the distal end of the pacing lead (1000).

In FIG. 13, it is assumed that the bipolar pacing leads of FIG. 12 aresubjected to a magnetic resonance imaging environment having anoperating radio frequency of approximately 64 MHz. As demonstrated inFIG. 13, the current (IRt1, which represents the current flowing throughtissue modeled resistor 1230 of FIG. 12) induced by the magneticresonance imaging environment and flowing through the tissue at thedistal end of the second bipolar pacing lead 1200 is reduced. In otherwords, the moving of the resonant circuits (2000 and 3000) to the distalend increases the effectiveness of the resonant circuits (2000 and3000), when a parasitic or distributive capacitance of the lead isinvolved.

It is noted that the current (IRt2, which represents the current flowingthrough tissue modeled resistor 1130 of FIG. 12) induced by the magneticresonance imaging environment and flowing through the tissue at thedistal end of the first bipolar pacing lead 1100 can have a magnitudebetween 1.21 and −1.21 amps.

However, space is very limited at the distal end of the lead. It isnoted that the inductor and capacitor values of the resonant circuits(2000 and 3000) can be adjusted which may help reduce the spacerequirement when implementing the resonant circuit.

The resonance frequency of the circuit is calculated by using theformula

$f_{RES} = \frac{1}{2\pi\sqrt{LC}}$

The required inductance (L) can be reduced (thereby reducing thephysical size required), by increasing the capacitance (C). So, forexample, if L=50 nH and C=123.7 pF, and if there is no room in thedistal end of the pacing lead for an inductor L having the inductanceL=50 nH, an inductor having an inductance of L=25 nH could be used ifthe capacitor used has a capacitance of 247.4 pF. The resonancefrequency remains the same.

To reduction of the heat generated by the induced current in the tissue,FIG. 14 provides a circuit representation of a bipolar pacing leadaccording to the concepts of the present invention. As illustrated inFIG. 14, the bipolar pacing lead 1000 includes two leads (1100 and1200). A first pacing lead 1100 includes resistance and inductancerepresented by a first resistor 1120 and a first inductor 1110,respectively. A second pacing lead 1200 includes resistance andinductance represented by a second resistor 1220 and a second inductor1210, respectively. At a distal end of each lead, the leads (1100 and1200) come in contact with tissue.

As illustrated in FIG. 14, the circuit paths from the distal ends of theleads (1100 and 1200) include a first tissue resistance, represented byfirst tissue modeled resistor 1130, and a second tissue resistance,represented by second tissue modeled resistor 1230.

The circuit representation of a bipolar pacing lead, as illustrated inFIG. 14, further includes a voltage source 300 that represents theinduced electromagnetic energy (voltage or current) from magneticresonance imaging, a body resistor 400 that represents the resistance ofthe body, and a differential resistor 500 that represents a resistancebetween the leads.

In addition to the elements discussed above, the circuit representationof a bipolar pacing lead, as illustrated in FIG. 14, includes a resonantcircuit (5000) in series or inline with one of the pacing leads, namelythe second lead 1200. The resonant circuit 5000 includes a RLC circuithaving an inductor 5110 in parallel with a current limiting resistor5130 and a capacitor 5120.

The lead 1200 together with the inline resonant circuit (5000) acts asan anti-antenna device, thereby reducing the magnitude of the currentinduced through the tissue at the distal end of the pacing lead (1200).

The current limiting resistor 5130 reduces the current in the resonantcircuit 5000 to make sure that the inductor 5110 is not damaged by toomuch current passing through it.

FIG. 15, it is assumed that the bipolar pacing leads of FIG. 14 aresubjected to a magnetic resonance imaging environment having anoperating radio frequency of approximately the resonance frequency ofthe resonant circuit 5000. As demonstrated in FIG. 15, the current(IRt1, which represents the current flowing through tissue modeledresistor 1230 of FIG. 14) induced by the magnetic resonance imagingenvironment and flowing through the tissue at the distal end of thesecond bipolar pacing lead 1200 can be greatly reduced, notwithstandingthe addition of the current limiting resistor 5130. It is noted that thecurrent (IRt2, which represents the current flowing through tissuemodeled resistor 1130 of FIG. 14) induced by the magnetic resonanceimaging environment and flowing through the tissue at the distal end ofthe first bipolar pacing lead 1100 can have a magnitude between 1.21 and−1.21 amps. This reduced magnitude of current (IRt1, which representsthe current flowing through tissue modeled resistor 1230 of FIG. 14) atthe distal end of the bipolar pacing lead can significantly reduce thedamage to the tissue due to heat generated by the current flowing to thetissue. It is noted that the current ILFilter1 is the current throughthe inductor 5110 when the resistor 5130 is a small value.

FIG. 16, it is assumed that the bipolar pacing leads of FIG. 14 aresubjected to a magnetic resonance imaging environment wherein theresistance of the current limiting resistor 5130 is increased. Asdemonstrated in FIG. 16, the current (IRt1, which represents the currentflowing through tissue modeled resistor 1230 of FIG. 14) induced by themagnetic resonance imaging environment and flowing through the tissue atthe distal end of the second bipolar pacing lead 1200 can be reduced,but the increased resistance of the current limiting resistor 5130 has aslight negative impact on the effectiveness of the resonant circuit5000.

It is noted that the current (IRt2, which represents the current flowingthrough tissue modeled resistor 1130 of FIG. 14) induced by the magneticresonance imaging environment and flowing through the tissue at thedistal end of the first bipolar pacing lead 1100 can have a magnitudebetween 1.21 and −1.21 amps. This reduced magnitude of current (IRt1,which represents the current flowing through tissue modeled resistor1230 of FIG. 14) at the distal end of the bipolar pacing lead cansignificantly reduce the damage to the tissue due to heat generated bythe current flowing to the tissue. It is noted that the currentILFilter1 through the inductor 5110 has decreased with the increase inthe resistance of resistor 5130, thereby illustrating controlling thecurrent through the inductor 5110 of the resonant circuit.

It is noted that the frequencies used in generating the various graphsare examples and do not represent the exact frequencies to be used inthe design and manufacturing of these circuits. More specifically, theexact frequencies to be used are governed by the Larmor frequency of theproton in the Hydrogen atom and the frequency of the radio frequency ofthe magnetic resonance imaging scanner.

The gyromagnetic ratio for the proton in the Hydrogen atom is γ=42.57MHz/T or γ=42.58 MHz/T, depending on the reference used. In thefollowing discussion γ=42.57 MHz/T will be used.

Given that the Larmor equation is f=B₀×γ, the frequency to which theresonant circuit is to be tuned, for example, in a 1.5 T magneticresonance imaging scanner, is f=(1.5 T)(42.57 MHz/T)=63.855 MHz.

The following table gives the resonance frequency for several casesalong with example circuit parameter values for the inductor andcapacitor to form the resonance circuit.

TABLE 1 Circuit Resonance B₀ Frequency Example Circuit Parameters(Tesla) (MHz) Inductor (nH) Capacitor (pF) 0.5 21.285 50 1118.2 1.042.57 50 279.55 1.5 63.855 50 124.245 3.0 127.71 50 31.06

These circuit parameter values are for the ideal case. It is expectedthat the actual values used in a real circuit could be different. Thatis, in the excitation signal's frequency environment of the magneticresonance imaging scanner, there are other effects (like parasiticcapacitance in the inductor) that may affect the circuit, requiring thecircuit parameters to be adjusted. In one embodiment, the resonantcircuits are tuned to a frequency close to the ideal values given inTable 1. In another embodiment, at least one resonant circuit is tunedto within 5 MHz of the ideal resonant frequency given in Table 1. Instill another embodiment, at least one resonant circuit is tuned towithin 10 MHz of the resonant frequency given in Table 1.

It is noted that introducing the resonant circuit only into one of thetwo bipolar pacing wires may result in an increase in the currentthrough the other wire.

For example, as illustrated in FIG. 17, when no resonant circuits areincluded with the bipolar pacing leads (FIG. 2), the current flowingthrough the first tissue modeled resistor 130 and second tissue modeledresistor 230 of FIG. 2 is significant, thereby generating heat topossibly damage the tissue.

On the other hand, as illustrated in FIG. 18, when a resonant circuit orresonant circuits are included in only one of the bipolar pacing leads(FIGS. 4, 7, 10 and 14), the current (IRt1) flowing through the secondtissue modeled resistor 1230 is significantly reduced in the one lead,but the current (IRt2) slightly increases in the first tissue modeledresistor 1130. It is noted that these behaviors are dependent on thecharacteristics of the implemented pacing lead and pulse generatorsystem.

On the other hand, as illustrated in FIG. 19, when a resonant circuit orresonant circuits are included in both bipolar leads (not shown), thecurrent (IRt1 and IRt2) flowing through the tissue modeled resistors 130and 1130 and second tissue modeled resistor 230 and 1230 issignificantly reduced.

It is noted that even if the resonant circuits of the present inventionare tuned, for example to 63.86 MHz on the bench top, when the resonantcircuits of the present invention are placed in the patient's body, theresonant circuits of the present invention may shift resonance a littlebecause of inductive and capacitive coupling to the surroundingenvironment.

Notwithstanding the potential shift, the concepts of the presentinvention still significantly reduce the heat generated current in thetissue at the distal end of the bipolar pacing leads, as illustrated inFIGS. 20 and 21. FIGS. 20 and 21 provide a graphical representation ofthe effectiveness of the resonant circuits of the present invention asthe circuits are tuned away from the ideal resonance of 63.86 MHz (forthe 1.5 T case).

In FIG. 20, the inductance of the resonant circuit is increased by 10%.In this instance, the resonant circuits of the present inventionsignificantly reduce the heat generated by currents (IRt1 and IRt2) inthe tissue at the distal end of the bipolar pacing leads.

Moreover, in FIG. 21, the inductance of the resonant circuit isdecreased by 10%. In this instance, the resonant circuits of the presentinvention significantly reduce the heat generated by currents (IRt1 andIRt2) in the tissue at the distal end of the bipolar pacing leads.

Therefore, the resonant circuits of the present invention need not beperfectly tuned to be effective. As mentioned above, even if theresonant circuits of the present invention were perfectly tuned, onceimplanted into a patient, the circuits are expected to shift resonancefrequency.

FIG. 22 illustrates an adapter, which can be utilized with an existingconventional bipolar pacing lead system. As illustrated in FIG. 22, anadapter 6000 includes a male IS-1-BI connector 6200 for providing aconnection to an implantable pulse generator 6900. The adapter 6000includes a female IS-1-BI connector 6500 for providing a connection tobipolar pacing lead 6800. The female IS-1-BI connector 6500 includeslocations 6600 for utilizing set screws to hold the adapter 6000 to thebipolar pacing lead 6800.

The adapter 6000 further includes connection wire 6700 to connect theouter ring of the bipolar pacing lead 6800 to the outer ring of theimplantable pulse generator 6900. The adapter 6000 includes a wire 6400to connect an inner ring of the bipolar pacing lead 6800 to a resonantcircuit 6300 and a wire 6100 to the resonant circuit 6300 to an innerring of the implantable pulse generator 6900. It is noted that anadditional resonant circuit could be placed between the outer ring ofthe bipolar pacing lead 6800 and the outer ring of the implantable pulsegenerator 6900.

It is noted that the resonant circuit 6300 in FIG. 22 can be multipleresonant circuits in series. It is also noted that the adaptor 6000 canbe manufactured with resonant circuits in series with both wires of thebipolar pacing lead. It is further noted that this adapter is connectedto the proximal end of the bipolar pacing lead.

Additionally, the adapter of the present invention may include enoughmass in the housing to dissipate the heat generated by the resonantcircuits. Alternatively, the adapter may be constructed from specialmaterials; e.g., materials having a thermal transfer high efficiency,etc.; and/or structures; e.g., cooling fins, etc.; to more effectivelydissipate the heat generated by the resonant circuits. Furthermore, theadapter may include, within the housing, special material; e.g.,materials having a thermal transfer high efficiency, etc.; and/orstructures; e.g., cooling fins, etc.; around the resonant circuits tomore effectively dissipate the heat generated by the resonant circuits.

The concepts of the adapter of FIG. 22 can be utilized in a differentmanner with an existing conventional bipolar pacing lead system. Forexample, an adapter may include a connector for providing a connectionto an implantable electrode or sensor. On the other hand, the adaptermay include an implantable electrode or sensor instead of a connectiontherefor.

The adapter may also include a connector for providing a connection tobipolar pacing lead. The connector may include locations for utilizingset screws or other means for holding the adapter to the bipolar pacinglead.

As in FIG. 22, this modified adapter would include a connection wire toconnect one conductor of the bipolar pacing lead to the electrode orsensor. The modified adapter would include a wire to connect the otherconductor of the bipolar pacing lead to a resonant circuit and a wire tothe resonant circuit to ring associated with the electrode of otherdevice associated with the sensor, such as a ground. It is noted that anadditional resonant circuit could be placed between the one conductor ofthe bipolar pacing lead and the electrode or sensor.

It is noted that the resonant circuit can be multiple resonant circuitsin series. It is also noted that the modified adaptor can bemanufactured with resonant circuits in series with both wires of thebipolar pacing lead. It is further noted that this modified adapter isconnected to the distal end of the bipolar pacing lead.

Additionally, the modified adapter of the present invention may includeenough mass in the housing to dissipate the heat generated by theresonant circuits. Alternatively, the modified adapter may beconstructed from special materials; e.g., materials having a thermaltransfer high efficiency, etc.; and/or structures; e.g., cooling fins,etc.; to more effectively dissipate the heat generated by the resonantcircuits. Furthermore, the modified adapter may include, within thehousing, special material; e.g., materials having a thermal transferhigh efficiency, etc.; and/or structures; e.g., cooling fins, etc.;around the resonant circuits to more effectively dissipate the heatgenerated by the resonant circuits.

In one embodiment, all other wires and electrodes, which go into amagnetic resonance imaging environment, (and not necessarily implantedinto the patient's body) can be augmented with a resonant circuit. Anywires to sensors or electrodes, like the electrodes of EEG and EKGsensor pads, can be augmented with a resonant circuit in series withtheir wires. Even power cables can be augmented with resonant circuits.

Other implanted wires, e.g. deep brain stimulators, pain reductionstimulators, etc. can be augmented with a resonant circuit to block theinduced currents caused by the excitation signal's frequency of themagnetic resonance imaging scanner.

Additionally, the adapter of the present invention, when used withinimplanted devices, may contain means for communicating an identificationcode to some interrogation equipments external to the patient's body.That is, once the implantable pulse generator, adapter, and pacing leadare implanted into the patient's body, the adapter has means tocommunicate and identify itself to an external receiver. In this way,the make, model, year, and the number of series resonance circuits canbe identified after it has been implanted into the body. In this way,physicians can interrogate the adapter to determine if there is aresonance circuit in the adapter which will block the excitationsignal's frequency induced currents caused by the magnetic resonanceimaging scanner the patient is about to be placed into.

Furthermore, the adapter of the present invention has the capability ofbeing tested after implantation to insure that the resonance circuit isfunctioning properly.

Since the present invention is intended to be used in a magneticresonance imaging scanner, care needs to be taken when selecting theinductor to be used to build the resonant circuit. The preferredinductor should not contain a ferromagnetic or ferrite core. That is,the inductor needs to be insensitive to the magnetic resonance imagingscanner's B₀ field. The inductor should also be insensitive to theexcitation signal's frequency field (B1) of the magnetic resonanceimaging scanner. The inductor should function the same in anyorientation within the magnetic resonance imaging scanner. This might beaccomplished putting the inductor (for the entire resonant circuit) in aFaraday cage.

The resonant circuit of the present invention could also be realized byadding capacitance along the bipolar pacing lead, as illustrated in FIG.23. In FIG. 23, capacitors 8000 are added across the coils of the pacinglead 7000.

With respect to FIG. 23, the adjacent coiled loops of the coiled wireprovide the capacitors for an RLC parallel resonant circuit. Theadjacent coiled loops can be adjusted by various means (coiling pitch,wire cross section geometry (square, rectangular, circular), dielectricmaterial between adjacent loops) to tune the coiled wire to have aself-resonance at or near or harmonic of the radio-frequency of themagnetic resonance imaging scanner.

As illustrated in FIG. 23, an oxidation layer or an insulating materialis formed on the wire resulting in essentially a resistive coating overthe wire form. Thus, the current does not flow through adjacent coilloop contact points, but the current instead follows the curvature ofthe wire. The parasitic capacitance enables electrical current to flowinto and out of the wire form due to several mechanisms, including theoscillating electrical field set up in the body by the magneticresonance imaging unit.

In pacing leads and some other leads, a coiled wire is used. A thininsulative film (polymer, enamel, etc.) is coated over the wire used toelectrically insulate one coiled loop from its neighboring loops. Thisforms an inductor. By inserting an appropriate sized capacitor 8000across multiple loops of the coiled wire, a parallel resonance circuitsuitable for reducing the induced current, in accordance with theconcepts of the present invention, can be formed.

FIG. 24 shows the temperature of the tissue at the distal end of a wirewherein the wire includes a resonant circuit at the proximal end (A);the wire does not include a resonant circuit (B); and the wire includesa resonant circuit at the distal end (C).

As illustrated in FIG. 24, the “Proximal End” case (A) (resonant circuitat proximal end) results in a higher temperature increase at the distalend than when the resonant circuit is located at the distal end (C). Inthe demonstration used to generate the results of FIG. 24, a wire of 52cm in length and having a cap at one end was utilized, resulting in adistributive capacitive coupling to the semi-conductive fluid into whichthe wires were placed for these magnetic resonance imaging heatingexperiments.

For the “Proximal End” case (A) (resonant circuit at proximal end) only,the resonant circuit was inserted 46.5 cm along the wire's length. Sinceno current at 63.86 MHz can pass through the resonant circuit, this setsany resonant wave's node at 46.5 cm along the wire. This effectivelyshortened the length of the wire and decreased the wire'sself-inductance and decreased the distributive capacitance. Thesechanges then “tuned” the wire to be closer to a resonance wave length ofthe magnetic resonance imaging scanner's transmitted radio frequencyexcitation wave resulting in an increase in the current at the distalend of the wire.

The effective length of the wire with the resonant circuit is now 46.5cm rather than the physical length of 52 cm. That is, the inductance andcapacitance of the wire is now such that its inherent resonancefrequency is much closer to that of the applied radio-frequency. Hence,the modeled current through the distal end into the surrounding tissueincreases from about 0.65 Amps when there is no resonant circuit at theproximal end (FIG. 25) to about 1.0 Amps when the resonant circuit isinserted at the proximal end of the wire (FIG. 26).

As illustrated in FIG. 27 (which is a close up of trace “C” in FIG. 24),when the wire includes a resonant circuit at the distal end, thetemperature rise is significantly less (about 0.9° C. after 3.75minutes). On the other hand, when the wire includes a resonant circuitat the proximal end, the temperature rise at the distal end is greater(See trace “A” in FIG. 24).

Experimental results with the resonant circuit at the proximal end of a52 cm long bipolar pacing lead did not demonstrate a significantaltering of the heating of the tissue at the distal end, as illustratedin FIG. 28. The attachment of the resonant circuit to the proximal endof a pacing lead places a wave node at the end of the pacing lead (noreal current flow beyond the end of wire, but there is a displacementcurrent due to the capacitance coupling to the semi-conductive fluid).That is, adding the resonant circuit to the proximal end of the pacinglead, which does not change the effective length of the pacing lead,does not change the electrical behavior of the pacing lead.

Now referring back to FIG. 15, it is noted that the current (Lfilter1)through the resonant circuit inductor 5110 of FIG. 14 is alsoillustrated. As can be seen, the current (Lfilter1) through the resonantcircuit inductor 5110 of FIG. 14 is larger than the original currentpassing through a prior art lead, as illustrated in FIG. 3. Although theheating of the tissue is significantly decreased with the addition of aresonant circuit, there still may be a problem in that inductors israted for a certain amount of current before the inductor is damaged.

In anticipation of a possible problem with using inductors not having ahigh enough current rating, the present invention may provide multipleresonant circuits, each resonant circuit being connected in seriestherewith and having the same inductor and capacitor (and resistor)value as the original resonant circuit.

As noted above, FIG. 7 illustrates an example of multiple seriallyconnected resonant circuits. Although previously described as havingvalues to create different resonance values, the resonant circuits 2000and 3000 of FIG. 7 may also have substantially the same resonance valuesso as to reduce the current flowing through any single inductor in theresonant circuits 2000 and 3000 of FIG. 7.

Moreover, in anticipation of a possible problem with using inductors nothaving a high enough current rating, the present invention may provideresonant circuits with inductors having larger inductive values. It isnoted that it may be difficult to implement an inductor having a largerinductive value in a small diameter lead, such as a pacing lead or DBSlead. In such a situation, the inductor may be constructed to be longer,rather than wider, to increase its inductive value.

It is further noted that the resonance values of the resonant circuits2000 and 3000 of FIG. 7 may be further modified so as to significantlyreduce the current through the tissue as well as the current through theresonant circuit's inductor. More specifically, the multiple resonantcircuits may be purposely tuned to be off from the operating frequencyof the magnetic resonance imaging scanner. For example, the resonancefrequency of the resonant circuit may be 70.753 MHz or 74.05 MHz.

In this example, when one resonant circuit of the multiple resonantcircuits is purposely not tuned to the operating frequency of themagnetic resonance imaging scanner, the current through the tissue isreduced, while the current through the resonant circuit's inductor isalso reduced. Moreover, when two resonant circuits of the multipleresonant circuits are purposely not tuned to the operating frequency ofthe magnetic resonance imaging scanner, the current through the tissueis further reduced, while the current through the resonant circuit'sinductor is also further reduced.

It is further noted that when the two (or more) resonant circuits arenot tuned exactly to the same frequency and not all the resonantcircuits are tuned to the operating frequency of the magnetic resonanceimaging scanner, there is significant reduction in the current throughthe tissue as well as the current through the resonant circuits'inductors.

In summary, putting the resonant circuit at the proximal end of a pacinglead may not reduce the heating at the distal end of the pacing to asafe level. However, placing the resonant circuit at the proximal end ofthe pacing lead can protect the electronics in the implanted pulsegenerator, which is connected at the proximal end. To protect thecircuit in the implanted pulse generator, a resonant circuit is placedat the proximal end of the pacing lead so as to block any inducedcurrents from passing from the pacing lead into the implanted pulsegenerator.

Since the current in the resonant circuit, when in the magneticresonance imaging scanner (or other radio-frequency field with afrequency of the resonant frequency of the circuit) may be larger thanthe induced current in the lead (or wire) without the resonant circuit,there may be some heating in the resistive elements of the resonantcircuit (in the wires, connection methods, inductor, etc.). Thus, itwould be advantageous to connect high thermal conductive material to theresonant circuit to distribute any heating of the circuit over a largerarea because heating is tolerable when it is not concentrated in onesmall place. By distributing the same amount of heating over a largerarea, the heating problem is substantially eliminated.

To distribute the heat, the inside of the pacing lead polymer jacket canbe coated with a non-electrical conductive material, which is also avery good thermal conductor, and this connected to the circuit.Moreover, filaments of non-electrically conductive but thermallyconductive material can be attached to the circuit and run axially alongthe inside of the pacing lead assembly.

As discussed above, a lead may include a conductor having a distal endand a proximal end and a resonant circuit connected to the conductor.The resonant circuit has a resonance frequency approximately equal to anexcitation signal's frequency of a magnetic-resonance imaging scanner.The resonant circuit may be located at the distal end of the conductoror the proximal end of the conductor. The resonant circuit may be: aninductor connected in parallel with a capacitor; an inductor connectedin parallel with a capacitor wherein a resistor and capacitor areconnected in series; an inductor connected in parallel with a capacitorwherein a resistor and the inductor are connected in series; an inductorconnected in parallel with a capacitor and connected in parallel with aresistor; or an inductor connected in parallel with a capacitor whereina resistor is connected in series with both the capacitor and inductor.

It is noted that a plurality of resonant circuits may be connected inseries, each having a unique resonance frequency to match various typesof magnetic-resonance imaging scanners or other sources of radiation,such as security systems used to scan individuals for weapons, etc. Itis further noted that the lead may include a heat receiving mass locatedadjacent the resonant circuit to dissipate the heat generated by theresonant circuit in a manner that is substantially non-damaging tosurrounding tissue. Furthermore, it is noted that the lead may include aheat dissipating structure located adjacent the resonant circuit todissipate the heat generated by the resonant circuit in a manner that issubstantially non-damaging to surrounding tissue.

It is also noted that the above described lead may be a lead of abipolar lead circuit.

Moreover, as discussed above, an adapter for a lead may include ahousing having a first connector and a second connector, the firstconnector providing a mechanical and electrical connection to a lead,the second connector providing a mechanical and electrical connection toa medical device, and a resonant circuit connected to the first andsecond connectors. The resonant circuit may have a resonance frequencyapproximately equal to an excitation signal's frequency of amagnetic-resonance imaging scanner. The resonant circuit may be aninductor connected in parallel with a capacitor or an inductor connectedin parallel with a capacitor and a resistor, the resistor and capacitorbeing connected in series.

It is noted that a plurality of resonant circuits may be connected inseries, each having a unique resonance frequency to match various typesof magnetic-resonance imaging scanners or other sources of radiation,such as security systems used to scan individuals for weapons, etc. Itis further noted that the adapter may include a heat receiving masslocated adjacent the resonant circuit to dissipate the heat generated bythe resonant circuit in a manner that is substantially non-damaging tosurrounding tissue. Furthermore, it is noted that the adapter mayinclude a heat dissipating structure located adjacent the resonantcircuit to dissipate the heat generated by the resonant circuit in amanner that is substantially non-damaging to surrounding tissue.

Furthermore, as discussed above, medical device may include a housinghaving electronic components therein; a lead mechanically connected tothe housing and electrically connected through the housing; and aresonant circuit, located within the housing, operatively connected tothe lead and the electronic components. The resonant circuit may have aresonance frequency approximately equal to an excitation signal'sfrequency of a magnetic-resonance imaging scanner. The resonant circuitmay be an inductor connected in parallel with a capacitor or an inductorconnected in parallel with a capacitor and a resistor, the resistor andcapacitor being connected in series.

It is noted that a plurality of resonant circuits may be connected inseries, each having a unique resonance frequency to match various typesof magnetic-resonance imaging scanners or other sources of radiation,such as security systems used to scan individuals for weapons, etc. Itis further noted that the adapter may include a heat receiving masslocated adjacent the resonant circuit to dissipate the heat generated bythe resonant circuit in a manner that is substantially non-damaging tosurrounding tissue. Furthermore, it is noted that the adapter mayinclude a heat dissipating structure located adjacent the resonantcircuit to dissipate the heat generated by the resonant circuit in amanner that is substantially non-damaging to surrounding tissue.

It is noted that although the various embodiments have been describedwith respect to a magnetic-resonance imaging scanner, the concepts ofthe present invention can be utilized so as to be tuned to other sourcesof radiation, such as security systems used to scan individuals forweapons, etc. In these instances, the frequency of an electromagneticradiation source is the “normal” frequency of an electromagnetic wave.Even if the electromagnetic wave is “circularly polarized,” it is notthe circular frequency, but the “normal” frequency.

In another example for reducing the heat generated by the inducedcurrent in the tissue, FIG. 29 provides a circuit representation ofanother bipolar pacing lead according to the concepts of the presentinvention. As illustrated in FIG. 29, the bipolar pacing lead includestwo lead conductors (2100 and 2200). A first pacing lead conductor 2100is segmented into at least four sections. It is noted that the pacinglead could be segmented into more segments, but four segments are beingused for this representation. Each section includes coilself-capacitance (OCoil-C1, OCoil-C2, OCoil-C3, and OCoil-C4). Inparallel with the coil self-capacitance, each section has a coilinductance (LOC1, LOC2, LOC3, and LOC4) and a coil resistance (OC-R1,OC-R2, OC-R3, and OC-R4). The first pacing lead 2100 is covered with apolymer jacket to prevent direct contact with the body, thus radialcapacitors (Jacket1, Jacket2, Jacket3, and Jacket4) are formed along thelength of the lead.

The second pacing lead conductor 2200 is segmented into at least foursections. It is noted that the pacing lead could be segmented into moresegments, but four segments are being used for this representation. Eachsection includes self-capacitance (ICoil-C1, ICoil-C2, ICoil-C3, andICoil-C4). In parallel with the coil self-capacitance, each section hasa coil inductance (LIC1, LIC2, LIC3, and LIC4) and a coil resistance(IC-R1, IC-R2, IC-R3, and IC-R4). The second pacing lead 2200 is coveredwith a polymer tube or coating, thus forming capacitors (IOC1, IOC2,IOC3, and IOC4) along its length with the inside surface of the outercoiled wire being one of the capacitor's conductive surfaces.

FIG. 29 further illustrates a sine wave voltage source MRI-RF to modelthe induced potentials in the body in which the lead is placed.Furthermore, FIG. 29 has not modeled the pulse generator, which isnormally connected to the leads in place of capacitors C1 and C2. Tworesistors, Ring-Liquid and Tip-Liquid, represent the electrodes incontact with the body. It is noted that Tip-Liquid represents the innercoiled wire in contact with the body, through which minimal magneticresonance imaging induced current flow is desired.

FIG. 29 also illustrates mutual inductive coupling (LOC1LIC1, LOC2LIC2,LOC3LIC3, and LOC4LIC4) between the outer coiled wire and the innercoiled wire. It is noted that the polarity of the coupling needs to beconsidered. This accounts for the left-right hand winding of the twocoiled wires. The outer coiled wire may be wound left-handed and theinner coiled wire might be wound right-handed. Alternatively, both mightbe wound left-handed. In one embodiment, the mutual inductive couplingcan be adjusted to control the magnetic resonance imaging inducedcurrents through the coiled wires.

FIG. 30 illustrates an embodiment of a bipolar pacing device 3000. Thebipolar pacing device 3000 includes an implantable pulse generator 3400connected to the bipolar pacing leads. The pacing leads are connected toa tip electrode 3100 and a ring electrode 3300. The tip electrode 3100may be corkscrew shaped.

The pacing leads are covered with a shrink wrap 3200. It is noted thatthe properties of the shrink wrap can be adjusted to reduce the magneticresonance imaging induced current through the lead. Adding the shrinkwrap material over the existing polymer jacket of the pacing lead, or bychanging the type and/or dielectric and/or resistive properties and/orthe thickness of the polymer jacket changes the capacitance ofcapacitors (Jacket1, Jacket2, Jacket3, and Jacket4) of FIG. 29.

In one embodiment, the parameters of the various elements of FIG. 29 canbe adjusted to change the lead's electrical properties, thereby changingthe effect of magnetic resonance imaging induced heating (current). Forexample, the capacitance of the capacitors (Jacket1 through Jacket4) canbe adjusted by changing the lead's outer polymer jacket. Morespecifically, the outer polymer jacket can be changed, thereby changingthe capacitance of the capacitors (Jacket1 through Jacket4), by changingthe material, the thickness, and/or dielectric properties.

In another embodiment, the inductance of inductors (LOC1 through LOC4)can be adjusted. More specifically, the inductance (per unit length) ofthe outer coiled lead can be changed by changing the number of windings,changing the number of filar in the wire used, changing the crosssectional profile of the wire used, and/or coating the wire withmaterial whose magnetic saturation is higher than the DC static fieldstrength used in the MRI scanner.

In another embodiment, the capacitance of capacitors (OCoil-C1 throughOCoil-C4) can be adjusted. More specifically, the inter-loop capacitanceof the outer coiled wire can be changed by changing the pitch of thecoiled wire, changing the cross sectional shape of the wire used,changing the number of filars used for the coiled wire, coating thewire, and/or coating filars comprising the wire with non-conductivematerials; e.g., dielectric materials.

In a further embodiment, the resistance of resistors (OC-R1 throughOC-R4) can be adjusted. More specifically, the resistance (per unitlength) of the outer coiled lead can be changed by changing the wirematerial, changing the number of filar in the wire used, and/or changingthe cross sectional profile of the wire used.

In a further embodiment, the capacitance of capacitors (IOC1 throughIOC4) can be adjusted. More specifically, the capacitance formed betweenthe inner coiled wire and the outer coiled wire can be changed bychanging the radial distance between the inner coiled wire's outerradius and the outer coiled wire's inner radius, changing the materialinterposed between the inner and the outer coiled wires, changing thepitch (number of loops) of the outer coiled wire, and/or changing thenumber of loops of the inner coiled wire.

In another embodiment, the mutual inductive coupling of inductors(LOC1LIC1 through LOCC4LIC4) can be adjusted. More specifically, themutual inductive coupling between the inner and outer coiled wires canbe changed by changing the coiling handedness of the inner and or theouter coiled wires, changing the number of coiling loops of the outercoiled wire, changing the number of coiling loops of the inner coiledwire, changing the material between the inner and outer coiled wires,and/or changing the material within or on the coiled inner wire.

In another embodiment, the inductance of inductors (LIC1 through LIC4)can be adjusted. More specifically, the inductance (per unit length) ofthe inner coiled lead can be changed by changing the number of windings,changing the number of filar in the wire used, changing the crosssectional profile of the wire used, and/or coating the wire withmaterial whose magnetic saturation is higher than the DC static fieldstrength used in the magnetic resonance imaging scanner.

In a further embodiment, the inter-loop capacitance of capacitors(ICoil-C1 through ICoil-C4) can be adjusted. More specifically, theinter-loop capacitance of the inner coiled wire can be changed bychanging the pitch of the coiled wire, changing the cross sectionalshape of the wire used, and/or changing the number of filars used forthe coiled wire, coating the wire and/or filars comprising the wire withnon-conductive materials, e.g. dielectric materials.

In a further embodiment, the resistance of resistors (IC-R1 throughIC-R4) can be adjusted. More specifically, the resistance (per unitlength) of the inner coiled lead can be changed by changing the wirematerial, changing the number of filar in the wire used, and/or changingthe cross sectional profile of the wire used.

With respect to the description of FIGS. 31-35, Table 1 gives themeasured bipolar pacing lead sample's DC resistance for the inner andouter conductors (coiled wires).

TABLE 1 Inner Coil Outer Coil DC Resistance DC Resistance Lead Label(Ohms) (Ohms) Closed Pitch #1 37.8 74.2 Closed Pitch #2 37.0 70.2 ClosedPitch #3 37.2 68.1 Medium Pitch #1 18.1 73.8 Medium Pitch #2 18.7 68.6Medium Pitch #3 18.3 69.8 Open Pitch #1 12.0 71.8 Open Pitch #2 18.667.6 Open Pitch #3 14.1 68.1

Moreover, each of the samples illustrated in FIGS. 31-35 were subjectedto the conditions listed in Table 2.

TABLE 2 Imaging Parameters Field Strength 1.5 Tesla Scanner Coil BodySequence FSE XL Imaging Plane Axial TE 16 ms TE² 30 ms TR 67EchoTrainLength 4 FOV 48 Slice Thickness 10 mm Spacing 2 mm Freq 256Phase 192 Phase FOV 1.0 NEX 1 Bandwidth 50 Body mass 79 kg Number ofSlices 20 thru the leads

These parameters provided an SAR level of 1.8528 W/kg. The scan time was2 minutes 52 seconds.

In FIG. 31, a conventional pacing lead was tested using the parametersin Table 2. As illustrated in FIG. 31, the temperature measured at thedistal end of the pacing lead was about 42° Celsius.

FIG. 32 illustrates three pacing leads with closed windings wherein coilC. #1 W.S.W. has a single layer of polymer shrink wrap as illustrated inFIG. 30. As illustrated in FIG. 32, the temperature measured at thedistal end of the pacing lead was between 21° Celsius and 21.8° Celsius.

FIG. 33 illustrates three pacing leads with medium-closed windingswherein coil Med. #3 W.S.W. has a single layer of polymer shrink wrap asillustrated in FIG. 30. As illustrated in FIG. 33, the temperaturemeasured at the distal end of the pacing lead was between 40° Celsiusand 55° Celsius.

FIG. 34 illustrates three pacing leads with open windings wherein coilO. #3 W.S.W. has a single layer of polymer shrink wrap as illustrated inFIG. 30. As illustrated in FIG. 34, the temperature measured at thedistal end of the pacing lead was between 40° Celsius and 60° Celsius.

FIG. 35 illustrates three pacing leads with open windings wherein coilC. #1 W.S.W. has a single layer of polymer shrink wrap as illustrated inFIG. 29; three pacing leads with open windings wherein coil Med. #3W.S.W. has a single layer of polymer shrink wrap as illustrated in FIG.29; and three pacing leads with closed windings wherein coil O. #3W.S.W. has a single layer of polymer shrink wrap as illustrated in FIG.29. As illustrated in FIG. 35, the temperature measured at the distalend of the pacing lead was between 20° Celsius and 60° Celsius.

In one embodiment, a possible solution to the pacing lead magneticresonance imaging heating problem is to tune the coiled windings to havea self-resonance frequency close to or at the resonance frequency (RFfrequency, operating frequency) of the magnetic resonance imagingscanner.

In one embodiment, another solution to the pacing lead magneticresonance imaging heating problem is to have some portion of the longcoiled wires include a self-resonance at the operating frequency of themagnetic resonance imaging scanner, preferably, close to the distal endof the coiled wire(s).

The self-resonance is formed by the coiled loops of the wire (providingan inductance) creating a distributive capacitance between adjacentloops, thus forming a RLC circuit with the capacitance in parallel withthe resistor and inductor. The coiled wire can be tuned by changing theloop-to-loop spacing (the pitch of the coiled wire) or by changing thematerial between the loops (change the dielectric material forming thedistributive capacitance.)

The capacitance (and inductance and resistance) of the coiled wire (or aportion of the coiled wire) can also be tuned to have a self-resonanceclose to or at the operating frequency of the magnetic resonance imagingscanner by changing the cross-sectional geometry of the wire used toform the coiled wire; i.e., a wire that has a square cross-section,rather than the typical round cross-section.

It is noted that pacemakers and other devices can create risks to theirpatients when exposed to magnetic resonance imaging by: excessiveheating of the device (multiple causes) capable of producinguncontrolled tissue heating and thermogenic damage; induced voltages inthe device that can interfere with organ function and device diagnosticand therapeutic capabilities; and/or magnetic resonance image disruptionand distortion that prevents the visualization of tissues “close” to thedevice.

While it is relatively easy to demonstrate a heating or induced voltageproblem, it is far more difficult to prove a solution to these problems,due to the complex and unpredictable nature, which includes factors suchas: RF field strength; patient position in the coil; type of imagingsequence; patient characteristics; duration of imaging procedure; bodystructure being imaged; lead design; specific type of medical device;lead orientation within patient; the degree of perfusion near thedevice; temperature measurement procedure; and respiratory phase.

Magnetic resonance imaging energy is coupled into conductive leads intwo ways, antennae effect and electrical potential induced within thebody (implant acts as an electrical “short circuit”). High electricalcurrent densities at the lead-tissue interface induce resistive heatingin tissue. However, tissue heating can be substantially reduced byincreasing the high frequency (i.e. 64 MHz) electrical impedance of thelead.

In one embodiment, the magnetic resonance imaging scanner's frequency isfixed. Thus, the lead's self-resonance frequency should be shifted bychanging coil inductance and capacitance properties.

More specifically, in one embodiment, changing the wire form designchanges the capacitance-inductance characteristics of the lead and itsimpedance. Moreover, it is noted that adding a discrete component, highfrequency resonator to the lead changes the capacitance-inductancecharacteristics of the lead and its impedance.

In one embodiment, lead design geometry has a strong influence onmagnetic resonance imaging induced heating at 1.5 Tesla. Thus, the leadheating can be reduced to acceptable levels by properly choosing wireform design geometry or using discrete component resonator.

It is further noted that minimally disruptive lead design can reducelead heating to acceptable levels. When implanted, these designs canprovide a greater margin of patient safety and/or allow a greater numberof patients access to magnetic resonance imaging. These designs can alsobe applied to other similar design conductive implants such as ICD andDBS leads, guidewires, catheters, etc.

In another embodiment of the present invention, a resonance tuningmodule is used in conjunction with implantable devices that incorporateone or more leads that may be subject to unwanted heating at the distaltip due to RF energy used in magnetic resonance imaging.

As illustrated FIGS. 36 and 37, Leads 12000 and/or 14000 may be singleor multiple wires, and each may comprise one or more single filar ormultifilar conductors, parallel or concentric, such as may be used inany of a variety of implantable devices used to sense conditions in thebody and/or stimulate tissues in the body.

Device 10000 of FIG. 36 (in this example a pacemaker) has an integratedresonance tuning module 18000 that in turn has a control and adjustmentsubsystem 16000; as such it is a newly designed system requiring thekind of design/development cycle and regulatory approvals typical for apacemaker. It will be obvious that device 10000 may be substituted for apre-existing implanted pulse generator, while utilizing the existingimplanted lead(s) thus not requiring explanation and replacement of thelead(s).

Device 20000 of FIG. 37 is a standard “off-the-shelf” pacemaker that isconnected via two short leads 24000 and 26000 to a standalone resonancetuning module implant 22000 that has a control and adjustment subsystem16000 that is identical to that found in device 10000. It will beobvious that device 22000 may be interposed between an existingimplanted pulse generator and its existing implanted leads with arelatively minor surgical procedure.

It is known that if the overall pacemaker system is properly tuned sothat the lead is “self-resonant” at the RF frequency of the magneticresonance imaging system (e.g. 64 MHz for a 1.5 T system) heating at thedistal end of the pacing lead will be significantly or completelyeliminated.

It is also known that the resonant frequency of the pacemaker system isinfluenced by the design, materials, and construction of the lead(s),the path the lead(s) take(s) in the body, the electromagneticcharacteristics of body tissues, the pulse generator lead(s) connect(s)to, and other factors. Thus it is impractical or impossible to create asingle design for a pacing lead that will be properly self-resonant onceimplanted in the body as part of a pacing system.

The purpose of this invention is to provide for a module that is eitherintegral to the pulse generator, or connected between it and thetraditional multifilar lead, such that the overall system may be tunedto be self-resonant in spite of the variables described above.

Once the connections are made and the surgery is completed, part of thesetup procedure for the system involves instructing the control andadjustment subsystem 16000 to iteratively test for the resonantfrequency of the system and adjust inductive and/or capacitive elementswithin the resonance tuning module 18000 or 22000 to reach the desiredresonant frequency and fix it permanently or until such time as it isdesired to be readjusted (e.g. to 128 MHz for a 3.0 T system).

FIG. 38 illustrates a conventional electronic device 30000 that includesa connection port 32000, a circuit 31000, and an electrical line thatelectrically connecting the connection port 32000 to the circuit 31000.An assembly 40000 that includes a wire 42000 and a connector 41000 canbe connected to the electronic device 30000 by connecting connector41000 to connection port 32000.

In one embodiment, assembly 40000 is a wire connection between anelectrical device (not shown) and the electronic device 30000. The wire42000 may pickup ambient electrical signals from the surroundingenvironment. These signals may be intended to be picked up by assembly40000 or these signals may be unintentionally picked up, i.e., noise, byassembly 40000.

FIG. 39 illustrates an insertion of resonant circuit adaptor 50000suitable for connecting to connector 41000 and connecting to connectionport 32000 which places a resonant circuit in series with electricalwire 42000 and circuit 31000. In this embodiment, the insertion of theresonant circuit adaptor 50000 converts the assembly 40000 into ananti-antenna for the frequencies to which the resonant circuit adaptor50000 is tuned. It is noted that the resonant circuit adaptor 50000 maybe tuned to be equal to a frequency of the undesirable signals in theenvironment or the resonant circuit adaptor 50000 may be tuned to afrequency that takes into account the environment (in vitro) in whichthe assembly 40000 is located; i.e., the resonant frequency of resonantcircuit adaptor 50000 may take into account the interaction of blood tothe resonant frequency of the in vitro assembly 40000.

The resonant circuit adaptor 50000 may also include multiple RLCresonant circuits in series, each tuned to a different frequency. Thussignals on the assembly 40000 of the frequencies, to which the adaptor50000 is tuned, are significantly blocked from reaching the circuit31000.

FIG. 40 illustrates another electronic device 30000 that includes aconnection port 32000 and a circuit 31000. The electronic device 30000further includes resonant circuits (32200 and 32400) and electricallines (32100, 32300, and 32500). The resonant circuits (32200 and 32400)are in series with connection port 32000 and circuit 31000. It is notedthat, in one embodiment, the resonant circuits (32200 and 32400) may betuned to difference resonant frequencies.

When assembly 40000 is connected to connection port 32000, the assembly40000 plus resonant circuits (32200 and 32400) may act as ananti-antenna for the frequencies to which the resonant circuits (32200and 32400) are tuned. Thus, signals, including intentional signals aswell as noise, that match the frequency of the resonant circuits (32200and 32400) which are picked up (or received) by the assembly 40000,either intentionally or unintentionally, are significantly reduced fromreaching circuit 31000.

It is noted that the resonant circuits (32200 and 32400) may be tuned tobe equal to frequencies of the undesirable signals in the environment orthe resonant circuits (32200 and 32400) may be tuned to a frequency orfrequencies that take into account the environment (in vitro) in whichthe assembly 40000 is located; i.e., the resonant frequencies ofresonant circuits (32200 and 32400) may take into account theinteraction of blood to the resonant frequency of the in vitro assembly40000.

In another embodiment, as illustrated in FIG. 41, an electronic device30000 includes a circuit 31000, a connection port 32000, and a switch33000. The switch 33000, which may be a manually operated switch or anautomatic switch, selects which resonant circuit (32250 or 32450) isconnected in series with the assembly 40000 and the circuit 31000.

The resonant circuit 32450 is connected to the switch 33000 byelectrical line 32800 and to the circuit 31000 by electrical line 32950.The resonant circuit 32250 is connected to the switch 33000 byelectrical line 32700 and to the circuit 31000 by electrical line 32900.The connection port 32000 is connected to the switch 33000 viaelectrical line 32600.

When the assembly 40000 is connected to the connection port 32000, theassembly 40000 plus series resonant circuit 32450 or the assembly 40000plus series resonant circuit 32250 forms an anti-antenna connected tothe circuit 31000. The anti-antenna significantly reduces signals at theresonant frequency of the resonant circuits (32250 or 32450) that arepicked up by the assembly 40000.

The switch 33000 can be operated to select different anti-antennafrequencies by connecting the different resonant circuits (32250 or32450 in series with assembly 40000 and circuit 31000.

It is noted that the resonant circuits (32250 and 32450) may be tuned tobe equal to frequencies of the undesirable signals in the environment orthe resonant circuits (32250 and 32450) may be tuned to a frequency orfrequencies that take into account the environment (in vitro) in whichthe assembly 40000 is located; i.e., the resonant frequencies ofresonant circuits (32250 and 32450) may take into account theinteraction of blood to the resonant frequency of the in vitro assembly40000.

It is noted that the resistor-inductor-capacitor (RLC) resonance circuitof the present invention may be constructed as a substantially cylindershaped component. Examples of a substantially cylinder shaped componentare illustrated in FIGS. 42-48. The substantially cylinder shapedcomponent may be cylinder shaped electrode, or an insert to be placedinline in a lead (pacing, DBS, pain relief, etc.).

The substantially cylinder shaped component may include an RLC circuithaving a resonance frequency wherein the resonance frequency may be theresonance frequency of a magnetic resonance imaging scanner, a harmonicof a magnetic resonance imaging scanner frequency, or other appropriatetuned or detuned frequency. The substantially cylinder shaped componentmay also include a connection points to attach an external lead wire tothe RLC resonance circuit wherein one connection point may be located onthe “proximal” side. In the case where the component is not itself theelectrode, an additional connection point may be located on the “distal”side of the component.

The substantially cylinder shaped component may further include throughhole(s) or other channels or conduits for making electrical contact withother lead wires on both sides of the component. The substantiallycylinder shaped component may include a device, system, or medium forcarrying any heat away from the component, thereby providing a heat sinkfunction.

FIG. 42 provides a diagram of the electrical features of thesubstantially cylinder shaped component 9100. In FIG. 42, an RLCresonance circuit 9112 connected to an electrode 9110. As illustrated inFIG. 42, the RLC resonance circuit 9112 includes an inductor 9114connected in parallel with a series circuit of a resistor 9118 andcapacitor 9116. It is noted that the actual configuration of the RLCresonance circuit 9112 is dependent upon the resonance or non-resonancespecifications for the circuit. The RLC resonance circuit 9112 isconnected to a lead 9122 and to the electrode 9110, via lead 9120.

In one embodiment, the substantially cylinder shaped component 9100 mayinclude a low pass filter built rather than the RLC resonance circuit.In this embodiment, a “ground” connection point is to be provided inaddition to the lead wire connections.

It is further noted that the electrode surface, associated withsubstantially cylinder shaped component 9100, may be one “plate” of thecapacitor 9116.

FIG. 43 is one embodiment of a substantially cylinder shaped componentimplemented with a resonance circuit. As illustrated in FIG. 43, asubstantially cylinder shaped component 9200 includes an outerconductive cylinder electrode 9210, a ring of dielectric material 9212,an inner conductive ring 9214 forming the second capacitor plate,insulative wire 9216 wrapped to form an inductor, and a non-conductivering spacer 9218 which forms a feed through channel allowing othercoiled wire 9220 to pass through.

FIG. 44 is a side view of an embodiment of a substantially cylindershaped component implemented with a resonance circuit. As illustrated inFIG. 44, a substantially cylinder shaped component 9300 includes anouter conductive cylinder electrode 9310, a ring of dielectric material9312, an inner conductive ring 9314 forming the second capacitor plate,insulative wire wrapped to form an inductor 9316, and a non-conductivering spacer 9318 which forms a feed through channel allowing othercoiled wire 9320 to pass through. A first end 9322 of the inductor coil9316 is connected to the inner conductive ring 9314 by conductiveconnection element 9342. This conductive connection element 9342 is alsoelectrically connected to connection tab 9344, which allows a wire to beelectrically attached to the component 9300. A second end 9324 of theinductor coil 9316 is connected to the outer cylinder conductor 9310 byconductive connection element 9340. Tabs 9330 and 9332 are used, forexample, to attach a polymer jacket tube (not shown) to each end of thecomponent 9300. The polymer jacket tube may be, e.g., the outer polymerjacket of a pacing lead, or the outer polymer jacket of a deep brainstimulation lead, etc.

FIG. 45 is a side view of another embodiment of a substantially cylindershaped component. As illustrated in FIG. 45, a substantially cylindershaped electrode 9400 comprising an outer cylinder conductive electrode9410, a volume 9412 in which a low pass filter (not shown) isimplemented. The volume 9412 is electrically connected to the outerelectrode 9410, connection tab 9414 for connecting to one of the lead'sconductive wires, connection tab 9416 for connecting to the ground wirein the lead, and a feed through 9420, through which other lead wires9422 may pass through the component 9400. Tabs 9430 and 9432 are used,for example, to attach a polymer jacket tube (not shown) to each end ofthe component 9400. The polymer jacket tube may be, e.g., the outerpolymer jacket of a pacing lead, or the outer polymer jacket of a deepbrain stimulation lead, etc.

FIG. 46 illustrates a plurality of substantially cylinder shapedcomponent connected along a medical device, such as a guidewire, pacinglead, or deep brain stimulation lead. As illustrated in FIG. 46, aplurality of electrical wires 9800 engage a proximal side of asubstantially cylinder shaped component 9500. The substantially cylindershaped component 9500 may be an actual electrode or tissue interface,associated with an electrode or tissue interface, or electroniccomponent with no electrode or tissue interface.

It is further noted that the substantially cylinder shaped component maybe located anywhere along a lead's length. In other words, it is alsonoted that the RLC or LC circuit, contained within the substantiallycylinder shaped component, may be contained in another type housing andmay be located anywhere along a lead's length.

Moreover, it is noted that any number of substantially cylinder shapedcomponents may be associated with a single lead. Furthermore, it isnoted that any number of RLC or LC circuits, contained within thesubstantially cylinder shaped component, may be contained in anothertype housing and may be associated with a single lead. In this example,the different RLC or LC circuits may be tuned to the same frequency,different frequencies, or any combination thereof.

A plurality of electrical wires 9810 engage a proximal side of asubstantially cylinder shaped component 9600 and a distal side of thesubstantially cylinder shaped component 9500. The substantially cylindershaped component 9600 may be an actual electrode or tissue interface,associated with an electrode or tissue interface, or electroniccomponent with no electrode or tissue interface.

A plurality of electrical wires 9820 engage a proximal side of asubstantially cylinder shaped component 9700 and a distal side of thesubstantially cylinder shaped component 9600. The substantially cylindershaped component 9700 may be an actual electrode or tissue interface,associated with an electrode or tissue interface, or electroniccomponent with no electrode or tissue interface.

Depending upon the functionality of the substantially cylinder shapedcomponents 9500, 9600, and 9700, all the wires or a portion of the wiresmay pass therethrough.

FIG. 47 is a cut (or end) view of one embodiment of a substantiallycylinder shaped component 9500 implemented with a resonance circuit. Asillustrated in FIG. 47, a substantially cylinder shaped component 9500includes an outer conductive cylinder electrode 9511, a ring ofdielectric material between an inner conductive ring 9515 and the outerconductive cylinder electrode 9511 to form a capacitor. Thesubstantially cylinder shaped component 9500 further includes aninductor 9512 and a resistor 9513 connected to one of the wires from theplurality of electrical wires 9800. The remaining wires, a plurality ofelectrical wires 9810, pass through a volume 9514.

The volume 9514 may be a polymer or other substance with predefinedchannels for the plurality of electrical wires 9810. The channels mayform a predefined pattern so as to reduce or eliminate electricalinterference or cross-talk.

Moreover, volume 9514 may be filled with a polymer or other substanceafter the plurality of electrical wires 9810 is located therein. In thisembodiment, the volume may have a skeletal structure to provide apredefined pattern for the plurality of electrical wires 9810.

Furthermore, the volume 9514 may be filled with a polymer or othersubstance that provides heat sink functionality for the inductor 9512and resistor 9513 circuit.

It is noted that the substantially cylinder shaped component 9500 ofFIG. 47 may include a low pass filter instead of the RLC resonancecircuit. IN this embodiment, the inductor 9512 and resistor 9513 arereplaced with a low pass filter and the conductive rings are replacedwith non-conductive components.

FIG. 48 is a cut (or end) view of another embodiment of a substantiallycylinder shaped component 9500 implemented with a resonance circuit. Asillustrated in FIG. 48, a substantially cylinder shaped component 9500includes an outer conductive cylinder electrode 9511, a ring ofdielectric material between an inner conductive ring 9515 and the outerconductive cylinder electrode 9511 to form a capacitor. Thesubstantially cylinder shaped component 9500 further includes aninductor 9512 and a resistor 9513 connected to one of the wires from theplurality of electrical wires 9800. The remaining wires, a plurality ofelectrical wires 9810, pass through a volume 9514.

The volume 9514 may be a polymer or other substance with predefinedchannels for the plurality of electrical wires 9810. The channels mayform a predefined pattern to reduce or eliminate electrical interferenceor cross-talk.

Moreover, volume 9514 may be filled with a polymer or other substanceafter the plurality of electrical wires 9810 is located therein. In thisembodiment, the volume may have a skeletal structure to provide apredefined pattern for the plurality of electrical wires 9810.

Furthermore, the volume 9514 may include a sub-volume 9516, which can befilled with a polymer or other substance that provides heat sinkfunctionality for the inductor 9512 and resistor 9513 circuit.

It is noted that the substantially cylinder shaped component 9500 ofFIG. 48 may include a low pass filter instead of the RLC resonancecircuit. IN this embodiment, the inductor 9512 and resistor 9513 arereplaced with a low pass filter and the conductive rings are replacedwith non-conductive components.

FIG. 49 illustrates a lead with an RLC circuit at distal tip tuned todesired resonant frequency. In this embodiment, a single conductive paththrough lead, which can be a multi-filar conductor, is illustrated, butit is understood that a RLC circuit can be applied to each conductivepath in the lead.

Moreover, it is noted that the tuning need not be perfect and that thetuning is with completed lead in blood (or blood substitute) rather thanin air. Furthermore, it is noted that the resistor R reduces the currentmaximum in the resonant circuit that helps protect the inductor L frombeing damaged by too high a current. In addition, it is noted that theresistor R and the inductor L may be thermally connected to heat sinksto remove or distribute or limit the amount of heat that may build up inthese components.

FIG. 50 illustrates a lead with multiple RLC circuits at distal end oflead, in series with one another each RLC circuit tuned to block adifferent RF frequency, e.g. 63.8 MHz and 127.6 MHz. In this embodiment,a single conductive path through lead, which can be a multi-filarconductor, is illustrated, but it is understood that a RLC circuit canbe applied to each conductive path in the lead.

Moreover, it is noted that the tuning need not be perfect and that thetuning is with completed lead in blood (or blood substitute) rather thanin air. Furthermore, it is noted that the resistor R reduces the currentmaximum in the resonant circuit, which helps protect the inductor L frombeing damaged by too high a current. In addition, it is noted that theresistor R and the inductor L may be thermally connected to heat sinksto remove or distribute or limit the amount of heat that may build up inthese components.

As illustrated in FIG. 51, the lead with multiple RLC circuitspositioned along the leads length. At least one at the distal end of thelead tuned to approximately the same resonant frequency. In this way,circuit #1 of FIG. 51 might reduce the induced current at the distaltip/tissue interface by 50% of the induced current that occurs withoutcircuits, while circuit #2 of FIG. 51 may reduce the remaining inducedcurrent by another 50% resulting in a total of 75% reduction.

Additionally, multiple circuits distributed along the length of the leadreduce the amplitude of the induced current that each circuit alonewould have to handle, thereby reducing the possibility that the inducedcurrent will exceed the component's rating.

As noted above, an example of multiple circuits distributed along thelength of the lead to reduce the amplitude of the induced current thateach circuit alone would have to handle is illustrated by FIG. 51, whichshows five RLC circuits separated by different intervals d1, d2, d3, d4along the length d0 of the pacing lead. In other words, the lead ispolyfurcated in that the lead is broken into multiple sections such thelead may not be necessarily continuous.

As illustrated in FIG. 52, a diode is placed at the distal end of thelead. The diode blocks any induced current from running from the distaltip toward the proximal end of the lead; however, the diode does notprevent a pacing pulse (or induced currents) from traveling from theproximal end of the lead to the distal end of the lead. Therefore, thediode blocks ½ of the magnetic resonance imaging RF induced current frompassing through the tip/tissue interface reducing the heating thatoccurs.

It is noted that one or more diodes can be utilized to prevent thecurrent in the resonant circuit from exceeding the rating of theinductor or the voltage from exceeding the rating of the capacitor.

More specifically, as illustrated in FIG. 53, if the voltage across theinductor is such that the inductor's rating would be exceeded, thediode's breakdown threshold would also be exceeded, thereby providing analternative path around the inductor such that the inductor isprotected. FIG. 53 illustrates an RLC circuit as illustrated in FIG. 49,wherein two diodes are connected back to back across the inductor L.

In a similar situation, FIG. 54 illustrates an RLC circuit asillustrated in FIG. 49, wherein two diodes are connected back to backacross the capacitor C. If the voltage across the capacitor C is suchthat the capacitor's rating would be exceeded, the diode's breakdownthreshold would also be exceeded, thereby providing an alternative patharound the capacitor C such that the capacitor C is protected.

It is noted that current limiting diodes or constant current diodes maybe used to limit the current in the resonant circuit or in the leadconductors itself.

It is further noted that when the lead wire is a coiled wire, thecoiling (pitch) of the wire and any insulating coating between loops ofthe wire are altered to adjust the self-resonance of the wire to be thedesired magnetic resonance imaging operating frequency. In other words,the impedance of the wire is adjusted to be large at the operatingfrequency of the magnetic resonance imaging scanner. In this embodiment,the coiled wire of the lead is considered to be divided into twolengths, as illustrated in FIG. 55.

As illustrated in FIG. 55, the first half of the length of the coiledwire may be adjusted (coiling pitch, insulating-dielectric materialspacers, other) to have a high impedance (self resonance) at theoperating frequency of a first magnetic resonance imaging scanner, whilethe second half of the length of the lead is adjusted to have a highimpedance (self resonance) at the operating frequency of a secondmagnetic resonance imaging scanner. Thus, the lead has high impedance attwo different frequencies, for example 64 and 128 MHz.

While various examples and embodiments of the present invention havebeen shown and described, it will be appreciated by those skilled in theart that the spirit and scope of the present invention are not limitedto the specific description and drawings herein, but extend to variousmodifications and changes thereof.

What is claimed is:
 1. A resonance tuning module comprising: a firstinterface configured to provide an electrical connection with animplantable medical device having electronics therein; a secondinterface configured to provide an electrical connection with animplantable lead; a control and adjustment subsystem configured todetermine a combined resonant frequency of the implantable medicaldevice and the lead after implantation; and an adjustable impedancecircuit comprising at least one of an adjustable inductive element or anadjustable capacitive element, wherein, based on the combined resonantfrequency of the implantable medical device and the lead determinedafter implantation, the control and adjustment subsystem adjusts atleast one of the adjustable inductive element or the adjustablecapacitive element to adjust an impedance of the adjustable impedancecircuit to change the combined resonant frequency of the implantablemedical device and the lead after implantation.
 2. The resonance tuningmodule as claimed in claim 1, wherein the control and adjustmentsubsystem operatively controls the impedance of the adjustable impedancecircuit to change the combined resonant frequency of the implantablemedical device and the lead to be substantially equal to a targetresonant frequency.
 3. The resonance tuning module as claimed in claim1, wherein the control and adjustment subsystem operatively controls theimpedance of the adjustable impedance circuit to change the combinedresonant frequency of the implantable medical device and the lead to besubstantially equal to a radio-frequency of a magnetic resonance imagingscanner.
 4. The resonance tuning module as claimed in claim 1, whereinthe control and adjustment subsystem operatively controls the impedanceof the adjustable impedance circuit to change the combined resonantfrequency of the implantable medical device and the lead to besubstantially equal to 64 MHz.
 5. The resonance tuning module as claimedin claim 1, wherein the control and adjustment subsystem operativelycontrols the impedance of the adjustable impedance circuit to change thecombined resonant frequency of the implantable medical device and thelead to be substantially equal to 128 MHz.
 6. The resonance tuningmodule as claimed in claim 1, wherein the control and adjustmentsubsystem operatively controls the impedance of the adjustable impedancecircuit to change the combined resonant frequency of the implantablemedical device and the lead to be slightly detuned from aradio-frequency of a magnetic resonance imaging scanner.
 7. Theresonance tuning module as claimed in claim 1, wherein the control andadjustment subsystem operatively controls the impedance of theadjustable impedance circuit to change the combined resonant frequencyof the implantable medical device and the lead to be slightly detunedfrom 64 MHz.
 8. The resonance tuning module as claimed in claim 1,wherein the control and adjustment subsystem operatively controls theimpedance of the adjustable impedance circuit to change the combinedresonant frequency of the implantable medical device and the lead to beslightly detuned from 128 MHz.
 9. The resonance tuning module as claimedin claim 1, wherein the adjustable impedance circuit includes a variableinductor and a variable capacitor.
 10. The resonance tuning module asclaimed in claim 1, wherein the adjustable impedance circuit includes anadjustable inductive circuit and an adjustable capacitive circuit. 11.The resonance tuning module as claimed in claim 1, wherein theadjustable impedance circuit includes a variable inductor, a variablecapacitor, and a variable resistor.
 12. The resonance tuning module asclaimed in claim 1, wherein the adjustable impedance circuit includes anadjustable inductive circuit, an adjustable capacitive circuit, and anadjustable resistive circuit.
 13. The resonance tuning module as claimedin claim 1, wherein the resonance tuning module is removably connectedto the implantable medical device via the first interface and removablyconnected to the implantable lead via the second interface.